Somehow I missed this one until just now…
Here's what I do, and it's not necessarily the way you should do it, but it's a start.
First, I have a document where I hashed out my initial query. This is where I put everything I write to reviewers. Every time I contact another one, I read through my previous draft and generally tweak it a bit. But sometimes I realize I've dropped something important, so I can go back and borrow from an old version.
My basic template:
Quote:
Hi <name>,
I'm the author of <book title>, which is about <logline (or expanded two-sentence version of logline)>. I'd love to send you a free review copy if you're interested.
The book has already received some great reviews, like this one:
<great review (don't forget to include the reviewer's name and credentials!)>
You can find out more about <book title> on the official website: <link>
If you're interested in a free review copy, just tell me where to send it and I'll mail one right away. I'm also up for providing a giveaway copy for the review post if you'd like.
Thanks for your consideration. Looking forward to hearing from you!
<sign your name and include your e-mail address>
I'm pretty sure this isn't the absolute best template, but it's worked for me. I've gotten about a 50% yes rate with variations on it out of almost thirty requests sent in the past month. Some of that may be that I'm contacting the right people, but either way, I'm not doing too bad with it. (Plus, my template is somewhat based on the one in 
The Well-Fed Self-Publisher, so maybe there's something to it.)
Another free tip: You'll see some places suggesting that you stamp your review copies with a stamp saying, "REVIEW COPY NOT FOR RESALE." You'll also find reviewers complaining about authors who do this. My solution? I sign review copies to the reviewer and add a "thanks for the review" kind of note. Now it's a keepsake.